This invention relates to amplitude modulating techniques and, more particularly, to the task of amplitude modulating a high powered laser beam.
There are several applications in which it is desirable to modulate a beam of electromagnetic radiation for transmission. One such application is to provide an amplitude modulated source of infrared radiation to be used as sensing beams for gathering three dimensional image information which can be retrieved and later processed to extract useful data from a sensed object. A particularly powerful image processor for analyzing such image data is disclosed in U.S. Pat. No. 4,167,728 to Sternberg and related improvements such as those disclosed in U.S. Ser. No. 73,818, filed Sept. 10, 1979, to Sternberg, now U.S. Pat. No. 4,322,716, which are hereby incorporated by reference. Where these sensors are designed to detect objects whch are far away, the source of the detection beam must be relatively powerful and the beam must be amplitude modulated within a prescribed frequency range to achieve optimum performance. Preferably, the detection beams are derived from a CO.sub.2 laser providing 4-40 watts of power and providing a beam of 5-7 millimeters in diameter.
One widely used technique for modulating light beams is through the use of electro-optical devices. Electro-optical modulation unfortunately entails bi-refringent crystals, polarization analyzers, and quarter-wave plates. The high drive powers (in order of 4-40 watts) required for electro-optical modulation of high power laser beams also leads to problems resulting from RF heating when modulation is carried out in the megahertz range.
Traveling wave acousto-optic modulators are, on the other hand, less expensive and simpler devices which require less drive power for achieving amplitude modulation of collimated laser radiation. In traveling wave acousto-optical modulators the laser beam is passed through a medium such as germanium which has an acoustic transducer mounted on one surface. The acoustic transducer is driven by a relatively high frequency source (typically 40-50 megahertz) which generates an acoustical wave which travels from one end of the medium and passes out of the other end. Modulation is accomplished by turning the acoustical drive source on and off. Modulation frequencies of up to tens of megahertz may be achieved if the laser beam is focused to a very small diameter.
Thus, it can be appreciated that while traveling wave acousto-optical modulation techniques may be valuable for low power laser beams, this approach is not practical for moderate to high power beams. The power density within the focal volume of the higher powered beams may readily exceed the damage threshold of the optical medium. It has been discovered that continuous wave CO.sub.2 lasers with several watts of output power may only be modulated to frequencies of no higher than about one megahertz using traveling wave acousto-optical devices. Even so, the modulation efficiency is very low, approaching only about 30%. Those skilled in the art will appreciate that this results in a very poor signal to noise ratio.